The present invention addresses the compositional needs of corrugated high-density polyethylene (HDPE) pipe utilized for drainage, irrigation, storm and sanitary sewer applications. Poor environmental stress crack resistance (ESCR) of corrugated high-density polyethylene has been an impediment in competing against Polyvinylchloride (PVC), concrete and corrugated metal pipe. The corrugated plastic pipe fabricated from high-density polyethylene often cracks before, during or shortly after being installed in a trench and back filled due to insufficient ESCR. This problem has caused the American Association of State Highway Transportation Officials (AASHTO) to include a required minimum ESCR. This invention discloses HDPE compositions and methods for formulating HDPE compositions that are melt blends of virgin, preprocessed, regrind and recycled HDPE and comply with the standards of AASHTO for corrugated polyethylene pipe. These standards include specifications for density, melt flow index (MI), flexural modulus, tensile strength and environmental stress crack resistance of the pipe compounds and are incorporated herein by reference.
The AASHTO standards for corrugated polyethylene pipe utilized for drainage, irrigation, storm and sanitary sewer applications typically require the pipe be fabricated from HDPE. Current AASHTO standards require the polyethylene compositions comply with cell classification of 335400C according to ASTM D-3350. The cell classification of 335400C requires a maximum Melt Flow Index (MI) at 190 degrees Centigrade as per ASTM D1238 of 0.4 grams per ten minutes, a density of 0.945 to 0.955 grams per cubic inch as per ASTM D1505, minimum flexural modulus of 110,000 pounds per square inch according to ASTM D790 and minimum tensile strength of 3,000 pounds per square inch according to ASTM D638 and a minimum environmental stress crack resistance of 24 hours determined by a notched constant tensile load (NCTL) of 15% of the yield stress of the polyethylene tested as per ASTM D5397. These polyethylene compositions have an additional AASHTO requirement requiring the addition of at least 2 percent by weight of carbon black particles for ultra-violet resistance.
Typically, corrugated polyethylene pipe manufacturers utilize specialty blow-molding grades of high-density polyethylene having a bimodal or multi-modal molecular weight distributions. Debras et al. in U.S. Pat. No. 6,218,472 disclosed such a polyethylene composition satisfies the current AASHTO standards by means of a multi stage polymerization. The disadvantage of this approach is that the pipe manufacturer typically pays a premium for as polymerized virgin corrugated pipe grade high-density polyethylene and can not easily modify the physical properties of the polyethylene composition to enhance the physical properties or processability in relation to the pipe size and profile shape. Ideally, the corrugated pipe manufacturers would prefer to purchase lower cost off specification, wide specification, reprocessed and recycled polyethylene components that they blend to meet the appropriate AASHTO standards. The prior art includes blending approaches. For example, Michie, Jr., U.S. Pat. No. 4,374,227, whereby medium density polyethylene pipe blends with improved low temperature brittleness properties and gloss are composed of HDPE, LLDPE and a carbon black concentrate. Michie, Jr. discloses a thermoplastic Medium Density Polyethylene (MDPE) composition having a nominal density of 0.926 to 0.940 grams per cubic centimeter. Unfortunately, this approach has the disadvantage of too low a density to meet the cell classification of 335400C according to ASTM D-3350 for corrugated and profile HDPE pipe. Similarly Boehm et al. in their U.S. Pat. No. 5,338,589 and Morimoto et al. in their U.S. Pat. No. 5,189,106 disclose MDPE having density ranges of 0.930 to 0.940 grams per cubic centimeter. Boehm et al. and Morimoto et al. both utilize specific and different two-stage polymerization processes to produce blending components for the resulting medium density polyethylenes. The disadvantage of this approach is that it is limited to medium density polyethylene and excludes the high-density polyethylene density range of 0.945 to 0.955 grams per cubic centimeter required for corrugated and profile polyethylene pipe. Su in U.S. Pat. No. 4,824,912 discloses terblends of a major portion of LLDPE and minor amounts of HDPE of low molecular weight and of HDPE having high molecular weight. This approach also has the same disadvantage of being limited to low and medium density polyethylene compositions.
The object of this invention is to disclose blends of commodity HDPE components that provide corrugated HDPE pipe compositions having a density range of 0.945 to 0.955 grams per cubic centimeter and a MI of less than 0.4 with enhanced stress crack resistance and processing characteristics. Generally, commercially available HDPE copolymers polymerized to produce blow-molding grades of HDPE are often utilized for corrugated pipe. Several commercially available HDPE copolymer blow molding grades similar to Chevron Phillips 5202 HDPE grade comply with AASHTO standards for density, MI, flexural modulus and tensile strength but fail the environmental stress crack resistance requirements for notched constant tensile load (NCTL) ASTM D5397. The low ESCR is due to their characteristic broad molecular weight distribution (MWD) that includes low molecular weight fractions.
A further object of this invention is to disclose blend compositions of virgin, off specification, wide specification, reprocessed and regrind polyethylenes that enhance stress crack resistance of polyethylene pipe blends by increasing the number of tie molecules and thereby decreasing the number of loose ends. The number of molecular loose ends is decreased by reducing number of shorter polyethylene molecules by melt blending HDPE with sufficiently high molecular weight to provide exceedingly high ESCR with low molecular weight HDPE components having narrow molecular weight distributions to provide improve processability. It is an additional object of this invention to disclose the specific physical properties required to select both the high molecular weight and the low molecular weight HDPE components so that the number of loose ends associated with the short molecules are minimized and the physical properties of the blend composition meets the desired performance standards.
It is a further object of this invention to disclose a lower cost and more flexible method of providing HDPE compositions for corrugated plastic pipe than as polymerized multimode polyethylenes. In this regard, the invention discloses a method of varying the composition of high density polyethylene components having sufficiently different values of density and melt index such that the density and melt index of the blended composition can be varied independently to attain enhanced physical properties and processability respectively while maintaining an enhanced environmental stress crack resistance.
It is an additional object of this invention to provide HDPE pipe material with:
Enhanced ESCR and long-term stress crack resistance by utilizing high molecular weight (HMW) HDPE component as the major component wherein the low molecular weight fraction is diminished with increased molecular weight.
Enhanced physical properties by utilizing low molecular weight HDPE homopolymer as a minor component having a characteristic narrow molecular weight distribution with higher density, flexural modulus and tensile strength.
Enhanced processability by utilizing low molecular weight HDPE copolymer as a minor component having a characteristic narrow molecular weight distribution devoid of short molecules and sufficiently high melt index to improve processability without dramatically greatly decreasing the ESCR.
It is the further objective of this invention is to provide the corrugated HDPE pipe and fittings manufacturers, the opportunity to vary the blend ratios of virgin, recycled, off specification, wide specification, reprocessed and regrind HMW and low molecular weight HDPE""s to obtain the required combination of physical and process properties of pipe and fittings. For example the pipe manufacturer may vary blend ratios to enhance 24-hour impact behavior of the pipe, ESCR and flexural stiffness by specific pipe diameter and corrugation design.
This invention provides the benefit of blending recycled, reprocessed, wide and off specification and virgin HDPE""s to provide corrugated HDPE pipe and fittings material compositions having enhanced physical properties and processing characteristics capable of meeting and exceeding AASHTO standards.